In the industrial production of certain chemicals, a desired product may preferably be produced at one temperature but then subsequently may preferably undergo separation operations or other handling at another temperature. For process simplification and capital savings, if possible it can be advantageous to perform such separation operations over the temperature difference involved. For instance, chlorine dioxide is typically prepared in a reaction liquor at elevated temperature but for safety reasons is subsequently handled in low concentration, lower temperature aqueous solution.
Chlorine dioxide offers advantageous properties for various industrial uses and is particularly desirable for use as an elemental chlorine free bleaching agent, such as in the pulp and paper industry, or as a disinfectant, as in water purification and the like. Chlorine dioxide however is very unstable and can decompose vigorously if certain temperature, pressure, and/or concentration limitations are exceeded. For these reasons, chlorine dioxide is usually generated at the point of use and must be handled carefully. And as mentioned, for handling purposes, it is generally prepared in low concentration aqueous solutions.
Chlorine dioxide is typically generated chemically from either chlorate (e.g. sodium chlorate) or chlorite (e.g. sodium chlorite) precursors. Although the former precursors are less expensive, chlorine dioxide production from them is generally economic only for large industrial applications since relatively complex, expensive production systems are required. For smaller applications (e.g. sterilizing medical equipment), chlorite precursors are generally employed.
Chlorine dioxide can be generated from reactions of chlorate precursor and an appropriate reducing agent, e.g. hydrochloric acid, sulphur dioxide, methanol, hydrogen peroxide, manganese porphyrin. In an exemplary industrial process, an aqueous stream of HCl-acidified sodium chlorate solution is prepared, allowed to react while under suitable control, and product chlorine dioxide is continually removed from the solution. Chlorine dioxide can be generated in numerous ways from reactions of chlorite precursor and a variety of other reactants, including oxidizing agents and acids.
Industrial systems for producing chlorine dioxide typically involve some gas phase production and handling during the production even though the final product is provided in solution form. For instance, dilute chlorine dioxide gas may be generated and then later absorbed into chilled water during production for immediate use and/or storage. Often, air dilution is required for safety reasons. And the reaction kinetics may not be ideal if reaction product can not practically be removed as quickly as desired in the generator. Further, chlorine gas is a side product of a competing reaction in the production process. While this competing reaction can be minimized and side product chlorine can be separated from the chlorine dioxide to a great degree, some chlorine typically remains as an undesirable impurity in the product chlorine dioxide solution. An exemplary integrated chlorine dioxide process for pulp bleaching applications is described in “Adopting The Integrated Chlorine Dioxide Process For Pulp Bleaching, To Comply With CREP Regulations”, IPPTA J. Vol. 21, No. 1, January-March, 2009, p 123.
U.S. Pat. No. 4,683,039 discloses a method for the formation of an aqueous product solution of chlorine dioxide involving generating a donor medium comprising chlorine dioxide in a chlorine dioxide generator and then transferring gaseous chlorine dioxide through a membrane by pervaporation to a recipient aqueous medium. The membranes must be sufficiently porous to permit the flow of gases therethrough but sufficiently hydrophobic to prevent the passage of aqueous solution therethrough. Expanded polytetrafluoroethylene was considered suitable. While offering certain advantages, the problem of chlorine contamination remained. Chlorine was either allowed to pass through as well into the product or it was suggested that the recipient medium be acidified to inhibit dissolution of chlorine therein. However, acidifying the recipient medium does not eliminate chlorine dissolution. Either way, costly and complicated subsequent treatment to remove chlorine may be required.
WO2008035130 discloses methods to prepare fluids containing pure chlorine dioxide which are not contaminated by the starting materials or the byproducts of the chlorine dioxide synthesis or to deliver pure chlorine dioxide into any medium capable of dissolving chlorine dioxide. The chlorine dioxide generated in the process is transported across a pore free polymeric membrane via selective permeation into the target medium. The methods may be suitable for syntheses which do not involve by-product chlorine gas and/or for certain applications such as disposable devices. The membranes discussed have very high selectivity for the byproducts because their permeability is at least 3 orders of magnitude lower for the contaminating components compared to that of chlorine dioxide. However, the membranes discussed, e.g. silicone rubber, are not suitable for reasons of corrosion resistance for use in applications involving long term exposure to corrosive chlorine.
The use of chlorine dioxide in various applications continues to increase. There thus still remains a need to develop better methods and systems for producing chlorine dioxide, particularly at the industrial level. The present invention addresses this need and provides other benefits as disclosed below.